﻿Adult morphology and redescription of Lestidiopsindopacificus (Ege, 1953), with comments on the features of related species (Teleostei, Aulopiformes, Paralepididae)

﻿Abstract Two specimens representing the first known adults of Lestidiopsindopacificus (Ege, 1953) are reported and described from Taiwan, and the validity and generic assignment of this species are confirmed. The origin of the pelvic fin directly below the dorsal-fin base shows that L.indopacificus belongs to the L.mirabilis species complex. It can be separated from its congeners by the position of the nostrils above the posterior end of the maxilla, the light body color with unevenly distributed melanophores in adults, and a distinct combination of meristic values and other morphological characteristics. New geographic records are reported for the two other current members of this species complex, L.mirabilis (Ege, 1933) and L.extremus (Ege, 1953). The diagnostic features that separate these three very similar species are discussed.


Introduction
The barracudina genus Lestidiops was established by Hubbs (1916) but was assigned by Harry (1953a) to one of three subgenera in Lestidium before being elevated again by Rofen (1966) to a full genus. Lestidiops belongs to the naked paralepidids (subfamily Lestidiinae), which have a scaleless body except for the lateral-line, a single row of short teeth on the gill rakers, and more than 77 vertebrae. Lestidiops can be separated from the other genera of Lestidiinae by the following combination of features: head, snout, and body short to moderately slender; nostrils situated slightly to well before a vertical drawn through posterior end of maxilla; light organs and luminescent duct absent; anus close to tip of adpressed pelvic fin; anal-fin rays 25-32; lateral-line usually incomplete (i.e. not reaching caudal-fin base, except in some species); well-developed ventral adipose fin present between anus and anal fin; and unevenly distributed melanophores to uniform black in adults (Harry 1953a;Rofen 1966;this study). There are about 25 nominal species of Lestidiops, but no conclusion has been reached as to how many are valid (Ho pers. data).
Recently, two adult specimens of barracudina collected from off southern Taiwan were observed to have the pelvic-fin origin below the dorsal-fin base and no light organs or luminescent duct. They were initially identified as Lestidiops mirabilis (Ege, 1933) by the authors; however, both specimens had a much lighter body color compared to the uniformly dark brown to black body of specimens of L. mirabilis. More detailed examination has revealed that these two specimens represented the first adults specimens ever recorded of another, little-known species, Lestidiops indopacificus (Ege, 1953).
Lestidiops indopacificus, originally Lestidium indopacificum, was described based on juveniles collected from the Indian Ocean (Ege 1953: fig. 30), with many larval specimens taken from widely scattered sites across the Indo-West Pacific. At 31.0 mm SL, the holotype has been the largest specimen available until now. Although many specimens in collections have been putatively identified as this species, it has not yet been possible to confirm these. Most literature records have been based on the original description, and others were likely misidentifications (Ho pers. data).
Lestidiops mirabilis, originally Paralepis mirabilis, was described by Ege (1933) solely on its holotype (whereabouts unknown), which had been collected from the Sulawesi Sea in the western Pacific. Ege (1953) provided more details of this species based on many juveniles (<47 mm SL) from the tropical Indo-Pacific and western Atlantic oceans. Adults of this species are rather rare, and only a few specimens are known from scattered localities (e.g., Harry 1953b;Uyeno et al. 1983;Gloerfelt-Tarp and Kailola 1984;Ho and Huang 2022). Some additional adult specimens from various localities were examined in the present study.
Recently, Ho and Huang (2022) provided the first description of an adult Lestidiops extremus (Ege, 1953) from the Philippines and compared it with L. mirabilis. Another specimen from Myanmar, representing the first record of L. extremus in the eastern Indian Ocean, is reported in the present work.

Identity as Lestidiops indopacificus
The holotype of Lestidium indopacificum (ZMUC P2329237) is in rather poor condition, with its dorsal fin missing entirely. Ventral adipose fins are present on the abdominal ridge and well developed between the anus and the AFO. The eyes are large, the snout is short, and the eye diameter is about half the snout length. There are 32 prepelvic vertebrae, 49 preanal vertebrae, and 80 total vertebrae counted directly from the translucent specimen. There is neither a light organ around the eye nor a luminescent duct inside the abdominal cavity. The anus is located close to the tips of the adpressed pelvic fins.
The original data for the holotype (31.0 mm SL) provided by Ege (1953) recorded the predorsal length as 53.9% SL and the prepelvic length as 54.8% SL, which implies that the VFO is slightly behind the DFO and under the dorsal-fin base. No drawing of the holotype was provided, but it is notable that the drawing of the largest specimen (21.1 mm SL) provided in Ege (1953: fig. 26) shows its VFO situated on about the same vertical line as its DFO, which is slightly different from the relative positions of the fin origins implied by Ege (1953) for the holotype. Rofen (1966) later assigned the species to Lestidiops, a conclusion accepted here. The total of 80 vertebrae excludes it from Paralepidinae. The position of the VFO slightly behind the DFO excludes it from Stemonosudis and Dolichosudis. The presence of ventral adipose fins and the position of the anus close to the tips of the adpressed pelvic fins exclude it from Macroparalepis, and the lack of light organs or a luminescent duct excludes it from Lestidium and Lestrolepis. The position of the nostrils above or before the posterior end of the maxilla and the presence of a small pelvic fin exclude it from Uncisudis.
If the inferred position of the VFO in the holotype is accepted as standard, L. indopacificus shares this feature only with L. mirabilis and, by this alone, these two species can be separated from all other nominal congeners. According to Ege (1953), L. indopacificus has 79-83 total vertebrae, 34-36 prehaemal vertebrae, and 29-32 anal-fin rays, data that match our two adult specimens well and which are clearly different from those of the two other current members of the L. mirabilis complex. In sum, all the available information allows the present specimens to be recognized as adults of L. indopacificus.

The Lestidiops mirabilis complex
The three species reported below share a distinctive characteristic, namely, the VFO lies clearly behind the DFO, either below the dorsal-fin base or slightly behind that point. Rofen (1966) included L. mirabilis and L. extremus in the extremus complex (= L. mirabilis complex herein). However, many members of the scaled genera in the Paralepidinae have in common the VFO behind the DFO and the VFO usually under or slightly behind the dorsal-fin base (Post 1987;Ho and Duhamel 2019). Members of Lestidium atlanticum complex, i.e. L. atlanticum Borodin, 1928, L. orientale Ho, Tsai & Li, 2019, and L. longilucifer Ho, Graham & Russell, 2020 have the VFO at about same vertical as the DFO, or with the VFO sometimes slightly before or behind the latter (Ho et al. 2020).
Based on our examination, the three current members of the L. mirabilis complex can be separated from other species of Paralepidinae by having 80-87 vertebrae in totalal, a single row of small teeth on the gill rakers (vs multiple rows of teeth, slender in many species), and a naked body except for the lateral-line scales (vs a body fully covered with scales). They can be separated from members of the L. atlanticum complex by the lack of a luminescent duct inside the abdominal cavity (vs a long duct extending to the gular region).

Lestidiops indopacificus
Body moderately long, strongly compressed, depth at pectoral fin 12.7-13.2 times in SL. Caudal peduncle short, its length 1.5-1.6 times eye diameter. Ventral adipose fin very weakly developed along abdominal ridge between pectoral and pelvic fins, better developed on margin between pelvic and anal fins. Anus situated above tip of appressed pelvic fin (smaller specimens with anus slightly before fin tip), distance from VFO to anus about 3.5-4.0 times in V-A.
Head moderately slender, long-triangular, its length 4.0-4.1 in SL; snout slender and pointed anteriorly, its length 1.7-1.8 in HL. Mouth terminal, moderately large, its gape extending to about 1.5 times eye diameter in front of eye. Lower jaw slightly upturned at tip, with small distal tab of fleshy tissue. Eye small, its diameter 6.0-6.2 in HL. No light organ around eye. First suborbital bone slender, fifth and sixth bones expanded posteriorly, and seventh small. Interorbital space narrow, its width 11.1-11.2 in HL; some straight ridges present on top of head and snout. Two nostrils located directly above posterior end of maxilla, latter extending to point about 2/3 eye diameter in front of eye. Numerous sensory canals on snout, check, operculum, and jaws; numerous sensory pores on dorsal surface of snout and lower surface of lower jaw.
Gill filaments present on all four gill arches. Fourth arch mostly connected to gill chamber wall by membranes. Pseudobranchs present, their anterior halves included in a deep pocket.
DFO slightly in front of VFO, predorsal length 1.7 in SL. Pectoral-fin base behind posterior margin of gill cover, upper end of fin base slightly below horizontal drawn through lower margin of eye; no small pocket behind fin base. VFO directly under dorsal-fin base, pre-pelvic fin length 1.6 in SL. No axial scale behind pelvic-fin base (probably lost during trawling). Anal fin originating in posterior fourth of body, preanal length 1.3 in SL. Dorsal adipose fin over rear portion of anal-fin base, its base length about equal to eye diameter.
Two or three small fangs at tip of upper jaw, followed by single row of numerous small, retrorse teeth, these gradually becoming smaller on posterior part of jaw. Vomerine teeth absent. One or two fangs (either depressible or fixed) at front end of each lower jaw, followed by two rows of fangs arranged in about 6 pairs (more teeth arranged irregularly on right jaw of larger specimen); those of inner row long with knife-like tip and depressible; those in outer row much shorter, curved, and fixed, slightly embedded in tissue. Two rows of fangs on each palatine with anteriormost teeth arranged in 4 widely spaced pairs, those in outer row long and depressible, those in outer row small  Shield-shaped gill rakers present on epibranchial, ceratobranchial, and hypobranchial parts of each gill arch, each raker with 3-5 (rarely fewer) small teeth on broad base, these teeth barely extending upward beyond margin of gill arch. Teeth on pharyngeal arch short, arranged in oval patch with about 4 rows in middle. Single row of small teeth on fifth ceratobranchial.
Body scaleless, except for single row of lateral-line scales extending from above pectoral girdle to point above approximately two-thirds length of anal-fin base. Lateralline scales slightly longer than wide, gradually becoming smaller and narrower posteriorly; 2 large pores on each upper and lower corner of most scales and 1 pore at front and 1 pore on each upper and lower corners of those scales on about posterior third of lateral line. Luminescent duct absent. Coloration. When fresh, body pale to light gray, somewhat translucent, unevenly covered with melanophores ( Fig. 1A, B), and with abdomen bright white. Dorsal fourth of body densely covered with tiny melanophores; slightly larger melanophores beneath extending to lateral line and further downward, gradually becoming scattered; lower third of abdomen pale with large unpigmented areas. Dense melanophores on dorsal surface of head, anterior snout and jaws, and gular region; rest of head pale, with scattered melanophores except in large unpigmented areas on cheek and operculum. Inner surface of operculum pale with large black patch; mouth cavity pale. Abdominal ridge covered with dense melanophores (fewer and scattered in small specimens). Pectoral, dorsal and pelvic fins pale, with scattered melanophores; anal fin covered with dense melanophores except for the paler basal region; caudal fin covered with dense melanophores. When preserved, body light yellow in general, with pigmentation pattern similar to fresh condition (Figs 1C, D, 2A-D). Peritoneal membranes and stomach black; intestine pale.
Distribution. According to Ege (1953), circumglobal, including the western Atlantic, Indian, and central Pacific oceans based on larvae and juveniles, but more or less restricted to tropical regions. Adult specimens recorded from Indonesia (Gloerfelt-Tarp and Kailola 1984), Hawaii (Harry 1953b), Australia (Ho and Huang 2022), and Suriname and French Guiana (Uyeno et al. 1983); newly reported from Okinawa Trough, Myanmar, and Mozambique in present study. Bathymertic range 424-1,100 m for adults.
Remarks. The information on this species in Tables 1, 2 combines literature data with measurements and counts from examination of voucher specimens in earlier reports (e.g., Harry 1953b; Ho and Huang 2022) and additional specimens. Although L. mirabilis is a widespread species, adults appear to be rare, with only about 10 specimens known in collections. The specimen from Mozambique has the most lateral-line scales (85, including 63 large + 22 small), whereas others have 73-80 (64-67 + 8-16).
The number of large scales is fairly constant among specimens, but because most individuals were collected by bottom trawl and either suffered some degree of skin damage or had their body partly broken, it is possible that small scales at the posterior end of the lateral line are missing in some.  (Ege, 1953), SAIAB 203471, ca Harry (1953b) counted 11-13 pectoral-fin rays in four adults, whereas 13-14 such fin rays were counted in the present material. The first pectoral-fin element of paralepidids has two rays that are closely attached to each other, as can be seen in stained specimens (Ho et al. 2019a); Harry (1953b) probably counted them as a single ray, and, if so, his count should have been 12-14 rays.
The number of gill rakers also shows a large variation. The specimen from Suriname has 44 gill rakers in total (12 on the upper limb + 32 on the lower), whereas other individuals have 53-61 gill rakers (10-14 + 39-51). Rofen (1966) noticed that the vertebral counts are different among specimens from the western Atlantic Ocean (81-83) and New Caledonia (86), which he concluded was due to lack of data, not to the existence of different geographic populations. Our only western Atlantic specimen also has 81 vertebrae, whereas three Pacific specimens have 86 or 87 vertebrae. Examination of more specimens may prove that there are consistent differences among populations.
Remarks. This species was originally described from a type series consisting of three larvae from Indonesia, and the only previously known adult was reported from the Philippines by Ho and Huang (2022). This is a very rare species, with only these four specimens and one additional adult from Myanmar known. This Myanmar specimen represents the first record in the eastern Indian Ocean and suggests that L. extremus may have a broader distribution range in the Indo-west Pacific region than has been suspected. Ege (1953) included four species in his Lestidium indopacificum-mirabilis group, including L. indopacificum, L. atlanticum, L. mirabilis, and L. extremum. He (Ege 1953: 139) distinguished these four nominal species by the proportions of the predorsal, prepelvic, and preanal (measured from tip of snout to anus) lengths, and by the length to the origin of the anal fin (= preanal length herein). He also showed, however, that three of them have similar counts of prehaemal vertebrae, total vertebrae, and anal-fin rays (Ege 1953: 140, no data available for L. extremum). The present examination of adults of all three species shows that these length proportions change with growth and become quite similar, even overlapping, as the more slender body shape of adults is attained. Apart from Lestidium atlanticum, which remains in that genus, the other three species were later reassigned to Lestidiops and are currently in the L. mirabilis species complex (Rofen 1966;Post 1972; this study). The differences between the adults of these three species can now be presented and are based both on the present observations and data from the literature.

Discussion
The snout is quite slender in L. indopacificus, moderate in L. mirabilis, and rather stout in L. extremus. The VFO is slightly but clearly behind the dorsal-fin base in L. extremus, whereas it is under the dorsal-fin base in L. indopacificus and L. mirabilis (Ho and Huang 2022;this study). In L. extremus, the posterior end of the maxilla extends nearly to a vertical drawn through the anterior margin of eye, whereas in the two other species, the maxilla only reaches a point well before the eye.
There are 10 dorsal-fin rays in all three species, except that Harry (1953a) counted nine in a specimen of L. mirabilis. There are 28 or 29 (29-31 in Ege 1953) anal-fin rays in L. mirabilis, 30 in L. indopacificus, and 30 or 31 in L. extremum. Examination of more adult specimens may reveal overlapping or coincident ranges of counts for these features.
Total gill raker counts for L. mirabilis are 44 (Atlantic specimen) or 53-61 (Indo-Pacific specimens) versus 34 total gill rakers in two adults of L. extremus and 42-44 in two adults of L. indopacificus. The gill rakers are quite closely spaced in L. mirabilis, widely spaced in L. extremus, and intermediate in L. indopacificus. On each gill raker there are usually three or four closely spaced teeth, which are quite long in L. extremus, only moderately so in L. mirabilis, and short in L. indopacificus.
Comparison of the proportional measurements of all three species in the L. mirabilis species complex (Table 1) shows that the head is smaller in L. extremus compared to the other two species. In addition, the predorsal length is small, and the prepelvic length larger, in L. extremus. While the distance between VFO and AFO (V-A) is clearly smaller in this species than in the other two, the distance between DFO and AFO (D-A) is large. The snout and both jaws are also shorter in L. extremus than in the other two species, and the pre-nostril length is smallest in L. extremus and longest in L. indopacificus, as reflected by the different positions of the nostrils in these species.
As for meristic values (Table 2), L. indopacificus has some clearly low vertebral counts  compared to the other two species, and the latter two counts are consistent with the relatively low lateral-line scale counts , which also readily separate L. indopacificus from the other two species. Lestidiops extremus has the lowest caudal vertebra count (CV 39-40), which clearly separates it from the other two species, and L. mirabilis has more PDV (35-38) and PDLL (36-39) than the other two species. Finally, L. mirabilis has a higher total count of lateral line scales (TLL 73-85) versus L. indopacificus (66-70) and L. extremus (68)(69)(70)(71).
It is notable that both adult specimens of L. indopacificus have their nostrils directly above the posterior end of the maxilla, which is different from the two other species (with nostrils situated well before the end of the maxilla), as well as from most other species of Lestidiops, although information on this is lacking for some species (Harry 1953a). The more posterior nostril position of L. indopacificus is reminiscent of Stemonosudis, which was defined by Harry (1953a) as having the laterally positioned nostrils distinctly behind the posterior ends of the maxillae but later revised by Rofen (1966) to "nostrils varying in position from before to slightly behind a vertical from posterior tip of maxilla." Ho et al. (2019b) found that members of the S. rothschildi species complex also have their nostrils situated above or clearly before the posterior ends of the maxillae. More investigation is needed to fully delineate the taxonomic boundary between Lestidiops and Stemonosudis.